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There aren't many Supreme Court Justices - or 85-year-olds for that matter - who can legitimately be described as rock stars. But it's hard to think of a more appropriate way to refer to Ruth Bader Ginsburg. Maybe super hero also comes close?

Appointed to the United States Supreme Court in 1993, Ginsburg was just the second woman to gain such a posting (there have been only two since then), which built upon her tireless work as a legal crusader for gender equality.

Although long-respected as a feminist leader and instigator of social change, the last few years have seen Ginsburg emerge as a genuine cultural icon. The Notorious RGB, they call her. A steely force for good embraced by a world sorely lacking them.

This "rock star" status was calcified by an Oscar-nominated documentary released last year, RGB, which became an unexpected hit at the box office. When Brett Kavanaugh went through his controversial Supreme Court nomination hearings in September 2018, Ginsburg's value as a liberal voice on the highest court of the most powerful country in the world became even more apparent.

And now the frilly-collared freedom fighter is the subject of a new biopic starring Oscar nominee Felicity Jones (The Theory of Everything, Star Wars: Rogue One) as the youthful Ginsburg and Armie Hammer (Call Me By Your Name) as her devoted husband and legal partner, Martin.

Armie Hammer as Marty Ginsburg and Felicity Jones as Ruth Bader Ginsburg in the movie On The Basis of Sex

Demonstrating the challenges Ginsburg faced as a law student at a time when barely any women were, the film also chronicles her life as a legal professor and young parent. But the main thrust of On The Basis of Sex concerns a landmark tax case fought by Ginsburg that challenged the way the courts legally discriminated, based on gender.

Jones tells TimeOut she felt the full weight of Ginsburg's legacy when she accepted the role.

"They were enormous shoes to fill, even though she has quite small feet," says Jones. "They were still massive shoes and it was really intimidating. I was taking on one of the most beloved and most iconic women in history. So I definitely didn't take it on lightly."

In taking the job, the questions provoked by the film's existence suddenly fell on Jones' shoulders.

"Who was this woman when she was in her 20s and her 30s? How did she become the icon that she is today? Because obviously we all know Ruth as she is now and she's an 85-year-old woman, but she didn't come out of the womb like that."

The screenplay for On The Basis of Sex was written by Daniel Stiepleman, Justice Ginsburg's nephew, who provided Jones with youthful footage of his aunt and helped facilitate a meeting.

"That early footage was really vital," says Jones. "And then obviously meeting Justice Ginsburg herself, who is a phenomenal human being. I think we were all quite nervous, you feel a little bit speechless when you first meet her, because you have so much respect for her integrity. And there aren't many people around at the moment in positions of power who have their principles intact. So at first you kind of want to curtsy or bow to the Queen."

Felicity Jones stars as Ruth Bader Ginsburg in the movie On The Basis of Sex

One especially resonant thread in the film sees Ginsburg inspired to fight for social change by the way her teenage daughter Jane (played by Cailee Spaeny) embraces the revolutionary spirit of the late 1960s.

"It was so interesting playing and navigating those scenes," says Jones. "I feel that her daughter is another catalyst, she forces her mother to confront herself. In some ways Ruth had to conform to the patriarchal 50s idea of how a woman was supposed to be. Then you see her daughter in that next generation; she doesn't have to play to those same rules. You can feel that society is shifting a little bit and she can do it a little bit more on her own terms."

Although historical events are being portrayed, Jones says the central theme of On the Basis of Sex couldn't be more relevant.

"Ruth Bader Ginsburg was fighting on every single front, not only because of her gender [but] because of her faith, because of where she was from. She had doors continually slammed in her face. Just how much strength and persistence it took. She didn't come from a privileged background. She was an outsider and look at where she is now. So you kind of come out going: 'Wow, there is hope for all of us, that we can affect change in the world and that if we don't like the way things are, we can come together and speak about it and do something to change it.'"

Lowdown:Who: Felicity JonesWhat: Stars as a young Ruth Bader Ginsberg in biopic On the Basis of SexWhen: In cinemas next Thursday